The present invention relates to a leveling agent composition for use in forming ferroelectric polymer films and the films formed therewith, in particular, ferroelectric polymer films suitable for use in data processing devices.
Ferroelectrics are a class of dielectric materials that can be given a permanent electric polarization by application of an external electric field. Use of ferroelectric materials in data processing devices is disclosed in U.S. Patent Application No. US 2002/0044480 to Gudesen et al., which is directed to a ferroelectric data processing device comprising a thin film of ferroelectric material as a data-carrying medium. The film may be inorganic, a ceramic material, a polymer, or a liquid crystal. Use of ferroelectric polymers in data processing devices is also described, for example, by Y. Tajitsu et al., in “Investigation of Switching Characteristics of Vinylidene Fluoride/Trifluoroethylene Copolymers in Relation to Their Structures”, (Japanese Journal of Applied Physics, Volume 26, pp. 554-560, 1987).
It is known that only certain vinylidene fluoride polymers are ferroelectric, and that the presence of ferroelectricity and other properties is due at least in part to polymer characteristics such as polymer composition, structure, molecular weight, molecular weight distribution, the thermal history of the film, and the solvent used to form the film. See, e.g., the Abstract of an article by Cho, in Polymer, Volume 15, p. 67 (1991). Recently, Tashiro et al., in Macromolecules, Volume 35, p. 714 (2002) performed a detailed structural analysis of the various vinylidene fluoride crystal morphologies. Vinylidene fluoride polymers occur in four distinct crystal morphologies, all monoclinic. Without intending to bound by theory, form I has essentially planar zigzag chains forming a polar structure in which CF2 dipoles are parallel to each other along the crystallographic b-axis. The chains are tightly packed and tend to form large crystals. In form II, the CF2 dipoles are packed in anti-parallel mode along the b-axis. Form II is therefore nonpolar and less tightly packed than form I. Form III is also a tightly packed polar unit cell, and is obtained by casting from highly polar (but not necessarily hydrogen bonding) solvents such as dimethylacetamide or dimethylformamide. Form III may also be obtained by annealing forms II or IV at high temperature. Finally, form IV is a polar structure in which the chains are packed in parallel mode. Form IV is also a desirable form from the standpoint of ferroelectric properties because it can interconvert with form II. Copolymers of vinylidene fluoride exhibit similar characteristics.
Ferroelectric polymer films may be formed by a variety of processes including by casting a composition comprising a ferroelectric polymer film precursor dissolved in a solvent onto a substrate, followed by removal of the solvent to produce the film. However, insufficient wetting of the substrate, compositional changes, and free energy gradients created by evaporation of the solvent can result in defects within the film, including orange peel and other defects that result from the formation of Bénard convection cells within the film as the solvent evaporates. See, for example, C. M. Hanson; P. E. Pierce; Cellular Convection in Polymer Coatings-An Assessment, 12 Ind. Eng. Chem. Prod. Res. Develop. 1973, p. 67.
In addition to Bénard convection cells, other variations in surface morphology can arise during the coating process, particularly in crystalline polymers. For example, during solvent evaporation, the surface of the film can have a surface free energy that is considerably higher than that of the original solution. The size of the critical nucleus for crystallite formation is usually correlated to the surface energy of the incipient film. Smaller numbers of relatively large, organized spherulitic crystal domains generally obtain in regions of high surface energy and larger numbers of small, less organized crystal domains arise in regions of low surface energy. In applications where ferroelectric materials are used in electronic devices in which the electrodes are in contact with the ferroelectric material, the crystal domain and electrode sizes should be such that the electrical signals obtained from polling different devices are similar. For example, a large number of small crystal domains, relative to the electrode size, have a statistically better chance of yielding substantially similar electrical signals from a plurality of device structures during polling than a smaller number of large crystal domains. Control of free energy gradients during film formation and annealing therefore influences device performance.
In addition to controlling film morphology, the ferroelectric film must adhere to the one or more surfaces with which it is in contact sufficiently strongly to prevent delamination during subsequent processing steps. Such delamination can, for example, result from thermal cycling, immersion in fluids, or mechanical stresses. It is therefore desirable that additives to the film provide improved adhesion as well as control of the film morphology.
Other desired improvements include reduced polling fatigue, as manifested by the diminution of the remnant polarization after repeated polling of the ferroelectric device.
Attempts to control or eliminate defects arising from no uniformities in the ferroelectric film, delamination or polling fatigue include using co-solvents to change the film drying rate, using wetting agents to promote more even wetting of the substrate, or using surfactants to produce a more even surface tension throughout evaporation and cure of a film. In U.S. Pat. No. 6,340,720 to Lin et al., leveling agents are used to reduce defects in coatings comprising polyvinylidene fluoride coagulated with latex. While use of such leveling agents may improve some properties, they may also adversely affect properties that are important to use of the ferroelectric polymer film in a data processing device. For example, solvents, surfactants, and other processing aids may inhibit adhesion of the film to the support onto which it is being formed, may promote the formation of undesirable polymer crystal morphologies, or have other adverse affects. Accordingly, there remains a need in the art for methods and compositions for the manufacture of ferroelectric polymer films, particularly films suitable for use in a data processing devices, that are highly reproducible and that allow for control of film properties.